Chuilin Lai

A review: carbon nanofibers from electrospun polyacrylonitrile and their applications

Carbon nanofibers with diameters that fall into submicron and nanometer range have attracted growing attention in recent years due to their superior chemical, electrical, and mechanical properties in combination with their unique 1D nanostructures. Unlike catalytic synthesis, electrospinning polyacrylonitrile (PAN) followed by stabilization and carbonization has become a straightforward and convenient route to make continuous carbon nanofibers. This paper is a comprehensive and state-of-the-art review of the latest advances made in development and application of electrospun PAN-based carbon nanofibers. Our goal is to demonstrate an objective and overall picture of current research work on both functional carbon nanofibers and high-strength carbon nanofibers from the viewpoint of a materials scientist. Strategies to make a variety of carbon nanofibrous materials for energy conversion and storage, catalysis, sensor, adsorption/separation, and biomedical applications as well as attempts to achieve high-strength carbon nanofibers are addressed.

Growth of carbon nanostructures on carbonized electrospun nanofibers with palladium nanoparticles.

This paper studies the mechanism of the formation of carbon nanostructures on carbon nanofibers with Pd nanoparticles by using different carbon sources. The carbon nanofibers with Pd nanoparticles were produced by carbonizing electrospun polyacrylonitrile (PAN) nanofibers including Pd(Ac)(2). Such PAN-based carbon nanofibers were then used as substrates to grow hierarchical carbon nanostructures. Toluene, pyridine and chlorobenzine were employed as carbon sources for the carbon nanostructures. With the Pd nanoparticles embedded in the carbonized PAN nanofibers acting as catalysts, molecules of toluene, pyridine or chlorobenzine were decomposed into carbon species which were dissolved into the Pd nanoparticles and consequently grew into straight carbon nanotubes, Y-shaped carbon nanotubes or carbon nano-ribbons on the carbon nanofiber substrates. X-ray diffraction analysis and transmission electron microscopy (TEM) were utilized to capture the mechanism of formation of Pd nanoparticles, regular carbon nanotubes, Y-shaped carbon nanotubes and carbon nano-ribbons. It was observed that the Y-shaped carbon nanotubes and carbon nano-ribbons were formed on carbonized PAN nanofibers containing Pd-nanoparticle catalyst, and the carbon sources played a crucial role in the formation of different hierarchical carbon nanostructures.

Photoluminescence anisotropy of uni-axially aligned electrospun conjugated polymer nanofibers of MEH-PPV and P3HT

We report the preparation of the uni-axially aligned nanofibers containing poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) or poly(3-hexylthiophene) (P3HT) using the electrospinning technique. The emission anisotropy of the aligned conjugated polymer nanofibers was investigated using the luminescence polarization spectroscopy. The results revealed that the photoluminescence of the aligned nanofibers was highly anisotropic with stronger emission along the nanofiber axis; in contrast, the photoluminescence of random nanofibers and thin films showed weak emission anisotropy. These results suggested that the electrospun conjugated polymer nanofibers had macromolecular chains preferably oriented along the nanofiber axis. The study provides new guidance in the development of unique optoelectronic devices based on electrospun nanofibers of conjugated polymers.

Effects of humidity on the ultraviolet nanosensors of aligned electrospun ZnO nanofibers

The uni-axially aligned ZnO nanofibers with diameters of ∼200 nm were prepared by electrospinning followed by calcination at 600 °C. The effects of humidity and UV illumination on the electrical properties of electrospun ZnO nanofibers were investigated in air under different levels of relative humidity (RH). In the air with low RH, the photocurrent of the electrospun ZnO nanofiber based device increased approximately three orders of magnitude under UV illumination compared with the value measured in the dark. Intriguingly, the photocurrent increased in the air with increase of RH initially; when the RH was above ∼50%, the photocurrent decreased gradually. In the recovery step when the UV illumination was off, a faster current decay was observed with the increase of RH. These were attributed to the competitive surface effects resulted from adsorption/desorption and the related ionization/dissociation of oxygen and water molecules on the photo-generated charge carriers.

SERS-active silver nanoparticles on electrospun nanofibers facilitated via oxygen plasma etching

Manipulating the interaction between inorganic building blocks and polymeric supporting materials is crucial in the fabrication and optimization of hybrid hierarchical nanostructures. Herein, oxygen plasma etching was used to modify electrospun nanofibers of poly(methyl methacrylate) (PMMA) for facilitating the growth of silver nanoparticles (Ag NPs). The PMMA nanofibers in the form of overlaid films, surface-decorated with Ag NPs, were explored as active substrates for surface-enhanced Raman scattering (SERS). Strong SERS enhancement was observed from the Ag NP–PMMA films, as well as individual nanofibers. Our work not only fabricated nanocomposite materials with controlled hierarchical structures and remarkable SERS performances, but also provided a versatile method in tuning interfacial interactions within nanostructured materials.

Electrospun anatase-phase TiO2 nanofibers with different morphological structures and specific surface areas

Electrospun anatase-phase TiO2 nanofibers with desired morphological structure and relatively high specific surface area are expected to outperform other nanostructures (e.g., powder and film) of TiO2 for various applications (particularly dye-sensitized solar cell and photo-catalysis). In this study, systematic investigations were carried out to prepare and characterize electrospun anatase-phase TiO2 nanofibers with different morphological structures (e.g., solid, hollow/tubular, and porous) and specific surface areas. The TiO2 nanofibers were generally prepared via electrospinning of precursor nanofibers followed by pyrolysis at 500°C. For making hollow/tubular TiO2 nanofibers, the technique of co-axial electrospinning was utilized; while for making porous TiO2 nanofibers, the etching treatment in NaOH aqueous solution was adopted. The results indicated that the hollow/tubular TiO2 nanofibers (with diameters of ~300-500 nm and wall-thickness in the range from tens of nanometers to ~200 nm) had the BET specific surface area of ~27.3 m(2)/g, which was approximately twice as that of the solid TiO2 nanofibers (~15.2 m(2)/g) with diameters of ~200-300 nm and lengths of at least tens of microns. Porous TiO2 nanofibers made from the precursor of Al2O3/TiO2 composite nanofibers had the BET specific surface area of ~106.5 m(2)/g, whereas porous TiO2 nanofibers made from the precursor of ZnO/TiO2 composite nanofibers had the highest BET specific surface area of ~148.6 m(2)/g.

Electrospun carbon nano-felt derived from alkali lignin for cost-effective counter electrodes of dye-sensitized solar cells

In this study, freestanding and mechanically flexible nano-felt consisting of electrospun carbon nanofibers (ECNFs) derived from alkali lignin with a BET specific surface area of ∼583 cm2 g−1 and average pore size of ∼3.5 nm was prepared and then surface-deposited with Pt nanoparticles (Pt NPs). Both nano-felts of ECNFs and ECNFs–Pt were studied as cost-effective counter electrodes of dye-sensitized solar cells (DSSCs). The energy-dispersive X-ray spectroscopy (EDS) results showed that the amount of Pt NPs in ECNFs–Pt nano-felt was ∼9.9 wt%, and the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) results indicated that the Pt NPs with small sizes in the range of 2–20 nm were randomly distributed on the surface of ECNFs. The electrochemical impedance spectroscopy (EIS) tests revealed that the ECNFs-based counter electrode had low charge transfer resistance (Rct = 1.94 Ω cm2), and the Rct value was reduced to 1.2 Ω cm2 upon surface-deposition of Pt NPs. The prototype DSSCs based on ECNFs and ECNFs–Pt counter electrodes exhibited comparable performances to the DSSC based on a conventional Pt counter electrode in terms of short circuit density (Jsc), open circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η).

Electrospun carbon nanofibrous mats surface-decorated with Pd nanoparticles via the supercritical CO2 method for sensing of H2

Carbon nanofibers (in the form of an overlaid mat) were prepared by electrospinning of polyacrylonitrile followed by stabilization in air and carbonization in argon. Pd nanoparticles were then deposited on the surface of nanofibers by using Pd(acac)2 as the precursor via the supercritical CO2 method followed by pyrolysis at 600 °C. Structural characterizations indicated that the Pd nanoparticles were composed of single-crystalline clusters with sizes of a few nanometers. The hydrogen sensing properties of electrospun carbon nanofibrous mats surface decorated with Pd nanoparticles were investigated. The results indicated that the resistivity of Pd-decorated nanofibrous mats decreased linearly with the increase of H2 volume fraction from 0 to 0.7 in H2/N2 mixture gas, and the response showed excellent reversibility.

Effect of FeCl3 on the morphology, wetting behavior, and stabilization/carbonization of polyacrylonitrile nanofibers prepared by electrospinning

Abstract Herein we report that the neat polyacrylonitrile (PAN) nanofibers and PAN/FeCl3 composite nanofibers were prepared by the materials-processing technique of electrospinning. The effect of FeCl3 amount on the morphology and wetting behavior of the resulting composite nanofibers were studied by scanning electron microscopy (SEM) and drop shape analysis (DSA), respectively. The SEM images indicated that the diameters of composite nanofibers decreased with increase of FeCl3 amount, and this was probably due to the conductivity increase of spinning solutions. The results acquired from DSA measurement showed that the addition of FeCl3 increased the surface contact angle of water deposited on composite nanofibers, suggesting the higher hydrophobicity and the lower wetting behavior. Additionally, the effect of FeCl3 on stabilization and carbonization of the PAN composite nanofibers were investigated. The structural and morphological evolutions of the stabilized and carbonized nanofibers were studied by Fourier transfer infrared (FTIR) spectra and SEM. The FTIR spectra showed that the presence of FeCl3 in composite nanofibers considerably varied the mechanisms of thermo-chemical reactions during the heat treatments. The SEM images revealed that there were conglutinations of nanofibers after carbonization, while the degree of conglutination for the carbonized PAN composite nanofibers with FeCl3 was higher than that for the carbonized neat PAN nanofibers.